The adrenaline-induced hyperpolarization, which was recorded in neurons of bullfrog paravertebral sympathetic ganglia by means of the sucrose gap technique, was antagonized by 1 mM 4-aminopyridine. The response was unaffected by drugs which influence intracellular Ca2+ movements or Ca2+-sensitive K+ conductances, i.e. 100 or 200 microM Cd2+, 60 microM dantrolene Na+, 10 mM tetraethylammonium bromide, 0.5-2.0 microM apamin or 70 microM (+)-tubocurarine chloride. The spontaneous, rhythmic hyperpolarizations which occur in ganglionic neurons in the presence of 5 mM caffeine and reflect activation of Ca2+-sensitive K+ conductances following mobilization of intracellular Ca2+, were examined by means of intracellular recording. These responses were often biphasic, comprising a transient rapid early phase and a slow late phase. Tetraethylammonium (10 mM) and 0.5-2.0 microM apamin antagonized the rapid early phase and 70 microM (+)-tubocurarine chloride antagonized both phases of the response. Neither phase of these spontaneous, rhythmic, caffeine-induced hyperpolarizations were affected by 1 mM 4-aminopyridine. Although the adrenaline-induced hyperpolarization was antagonized by 50 microM 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate and by 50 microM quinidine, the majority of the results argue against the hypothesis that mobilization of intracellular Ca2+ is required for activation of the K+ conductance thought to underlie the adrenaline-induced hyperpolarization.